The Third Generation Partnership Program (3GPP) Release (Rel) 13 feature License Assisted Access (LAA) allows Long Term Evolution (LTE) equipment to also operate in the unlicensed 5 gigahertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. A future Rel-14 work item will add uplink transmissions to LAA. Accordingly, devices connect in the licensed spectrum (Primary Cell (PCell)) and use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum (Secondary Cell (SCell)). Standalone operation of LTE in unlicensed spectrum is also possible and is under development by the MuLTEfire Alliance.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) method needs to be applied. LBT involves sensing the medium for a predefined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
LTE
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as single-carrier Frequency Division Multiple Access (FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of Single Carrier FDMA (SC-FDMA) symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length TSUBFRAME=1 ms as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and are used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3. The reference symbols shown there are the Cell specific Reference Symbols (CRSs) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Uplink transmissions are dynamically scheduled, i.e., in each downlink subframe the base station transmits control information about which terminals should transmit data to the enhanced or evolved Node B (eNB) in subsequent subframes, and upon which resource blocks the data is transmitted. The uplink resource grid is comprised of data and uplink control information in the Physical Uplink Shared Channel (PUSCH), uplink control information in the Physical Uplink Control Channel (PUCCH), and various reference signals such as Demodulation Reference Signals (DMRSs) and Sounding Reference Signals (SRSs). DMRSs are used for coherent demodulation of PUSCH and PUCCH data, whereas SRS is not associated with any data or control information but is generally used to estimate the uplink channel quality for purposes of frequency-selective scheduling. An example uplink subframe is shown in FIG. 4. Note that uplink DMRS and SRS are time-multiplexed into the uplink subframe, and SRSs are always transmitted in the last symbol of a normal uplink subframe. The PUSCH DMRS is transmitted once every slot for subframes with normal cyclic prefix, and is located in the fourth and eleventh SC-FDMA symbols.
From LTE Rel-11 onwards, downlink or uplink resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH) is available. Resource grants are User Equipment (UE) specific and are indicated by scrambling the Downlink Control Information (DCI) Cyclic Redundancy Check (CRC) with the UE-specific Cell Radio Network Temporary Identity (C-RNTI) identifier.
LTE Uplink Timing Advance
Uplink transmissions from different UEs will arrive at different times at the eNB due to differences in propagation delay. In order to maintain uplink orthogonality, a UE-specific uplink Timing Advance (TA) is indicated to UEs in order to align the reception times of their transmissions at the eNB. The uplink TA is specified relative to the downlink reception timing for the UE. For initial access, the TA is computed by the eNB after a preamble transmission step by the UE. After initial access, the eNB can readjust uplink TA with TA commands. The TA can be configured by the eNB using an 11-bit command with a granularity of 0.52 μs, from 0 up to a maximum of 0.67 ms. An illustration of uplink TA is shown in FIG. 5, where UE 1 and UE 2 apply a TA of twice their respective one-way propagation delays from the eNB, such that both uplink transmissions are aligned in time when received by the eNB.
The eNB configures a timer for each UE, which is restarted by the UE each time a TA update command is received. If the UE does not receive another TA update command before the timer expires, it must then consider its uplink to have lost synchronization. In such a case, the UE is not permitted to make another uplink transmission of any sort without first transmitting a random access preamble to reinitialize the uplink timing.
CA
The LTE Rel-10 standard supports bandwidths larger than 20 megahertz (MHz). One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of CA. CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 6. A CA-capable UE is assigned a PCell which is always activated, and one or more SCells which may be activated or deactivated dynamically.
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case where the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
WLAN
In typical deployments of WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a Clear Channel Assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
A general illustration of the LBT mechanism of Wi-Fi is shown in FIG. 7. After a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the Acknowledgement (ACK) frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing an LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as Distributed Inter-Frame Space (DIFS)) after the channel is observed to be occupied before assessing again whether the channel is occupied. Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle, then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
In the above basic protocol, when the medium becomes available, multiple Wi-Fi stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW]. The default size of the random backoff contention window (CWmin) is set in the IEEE specs. Note that collisions can still happen even under this random backoff protocol when there are many stations contending for the channel access. Hence, to avoid recurring collisions, the backoff contention window size (CW) is doubled whenever the station detects a collision of its transmission up to a limit (CWmax), also set in the IEEE specs. When a station succeeds in a transmission without collision, it resets its random backoff contention window size back to the default value (CWmin).
LAA to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited, which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 8, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application a SCell in unlicensed spectrum is denoted as an LAA SCell.
LBT in 3GPP Rel-13 LAA
In Rel-13 LAA, LBT for downlink data transmissions follow a random backoff procedure similar to that of Wi-Fi, with Contention Window (CW) adjustments based on Hybrid Automatic Repeat Request (HARQ) Negative Acknowledgement (NACK) feedback. Several aspects of uplink LBT were discussed during Rel-13. With regard to the framework of uplink LBT, the discussion focused on the self-scheduling and cross-carrier scheduling scenarios. Uplink LBT imposes an additional LBT step for uplink transmissions with self-scheduling, since the uplink grant itself requires a downlink LBT by the eNB. The uplink LBT maximum CW size should then be limited to a very low value to overcome this drawback, if random backoff is adopted. Therefore, Rel-13 LAA recommended that the uplink LBT for self-scheduling should use either a single CCA duration of at least 25 μs (similar to downlink Dedicated Reference Signal (DRS)), or a random backoff scheme with a defer period of 25 μs including a defer duration of 16 μs followed by one CCA slot, and a maximum contention window size chosen from X={3, 4, 5, 6, 7}. These options are also applicable for cross-carrier scheduling of uplink by another unlicensed SCell.
A short uplink LBT procedure for the case involving cross-carrier scheduling by a licensed PCell remains open for further study. The other option on the table is a full-fledged random backoff procedure similar to that used by Wi-Fi stations.
Finally, the case of uplink transmissions without LBT when an uplink transmission burst follows a downlink transmission burst on that respective carrier (with a gap of at most 16 μs between the two bursts) was left open for further study in Rel-14.
An example to illustrate uplink LBT and uplink transmission when the uplink grant is sent on an unlicensed carrier is provided in FIG. 9.